I mainly work in React. You can write semantic HTML up to a point. Things like article, section, header, footer, etc. are supported. But if you wanted to make up your own tag and expect it to act like a div, I think React would complain that it doesn't understand that tag.

That's what I meant by "the promise of the semantic web". As I originally understood it, you could make your own custom tags that describe the intent of the document. You wouldn't be bound by what the w3c or browser vendors blessed. Any tags that were unknown to the browser would be treated mostly the same as a div. Web components occupy a similar space.